DE102018210720A1 - Hydrostatic drive with pressure cut-off and method for calibrating the pressure cut-off - Google Patents

Abstract

Disclosed is a hydrostatic travel drive with a hydraulic pump for supplying pressure medium to a hydraulic motor of the travel drive, the hydraulic pump having an actuating cylinder with at least one cylinder space and a stroke volume adjustable above it, and at least one electrically controllable pressure valve is provided, via which the cylinder space can be acted upon with an adjusting effective regulating pressure , In addition, the drive system has a device that can be used to limit the pressure of the hydraulic pump by influencing the signal pressure. A method for controlling the drive system is also disclosed.

Description

The invention relates to a hydrostatic drive with pressure cutoff according to the preamble of patent claim 1, and to a method for calibrating the pressure cutoff according to patent claim 9.

A generic hydrostatic travel drive has a hydraulic pump and a hydraulic motor which can be supplied with pressure medium by the latter, in particular in a closed hydraulic circuit. According to the applicant's data sheet RG-E 92003, a swash plate type axial piston variable displacement pump is known, the pressure of which it provides to the hydraulic circuit can be controlled directly. The hydraulic pump is physically designed such that the pressure always counteracts an actuating pressure of an actuating cylinder acting on the stroke volume of the hydraulic pump. As a result of the construction, the hydraulic pump has an internal control loop, as a result of which the pressure always acts in the direction of its own reduction. In pump operation of the hydraulic pump, it is effective in the direction of a reduction and in motor operation in the direction of an increase in the stroke volume. One chamber of the actuating cylinder is assigned to the traction mode and another, counteracting chamber to the towing or braking mode of the travel drive. By measuring the hydraulic pump with regard to its parameters pressure, signal pressure, displacement volume and speed, a map of the hydraulic pump is known, from which a necessary signal pressure can be determined in accordance with an accelerator pedal or driver request. This is done via an electronic control unit. This control of the hydraulic pump makes it possible to directly assign a drive torque to a position of the accelerator or accelerator pedal, which the operator perceives as very direct and therefore easily calculable control of the drive system.

To safeguard the pressure or high-pressure working lines, pressure relief valves are provided which release pressure medium from the relevant working line from a set limit. However, since this is energetically disadvantageous, a so-called pressure cut-off is provided approximately 30 bar below the pressure set on the pressure limiting valve. The pressure cut-off is implemented in such a way that a separate, smaller-sized pressure relief valve is provided, which is acted upon in the opening direction by the highest of the pressures in the working line and in the closing direction by the setpoint. If the working pressure reaches the setpoint or cut-off value, a pressure relief line, in which control pressure medium is provided at a pressure of approximately 30 bar, is relieved via this pressure relief valve. Since a control pressure of the respective chamber is reduced from the control pressure medium provided via pressure reducing valves, the maximum control pressure which can be provided also decreases in this way. Accordingly, the displacement volume of the hydraulic pump swings back in pump operation due to the lower signal pressure, as a result of which the pressure or working pressure is limited by the lower delivery volume of the hydraulic pump. This conventional limitation or pressure cut-off is therefore based on a hydromechanical control circuit with the pressure relief valve mentioned as the hydromechanical controller.

The disadvantage of this solution is the comparatively high expenditure on device technology for determining the highest of the pressures in the working lines, the provision of the pressure relief valve for pressure cutoff, and the energy loss due to the control pressure medium being blown off. In addition, the above-mentioned combination of an electronically controlled adjustable hydraulic pump with hydromechanically regulated pressure cutoff in transients can be difficult or unmanageable.

In the alternative case of an electronically controlled pressure cut-off based on pressure values recorded by pressure sensors, this proves to be complex and prone to vibration. This type of pressure cut-off also proves to be low-performing, since it reacts unintentionally to pressure peaks, and thus intervenes in the pressure cut-off even in uncritical operating states and thus reduces the signal pressure and the pump volume. This can cause vibrations. If, for example, the pressure sensor system also fails, the pressure cutoff also fails, which leads to an energetic disadvantage at the latest when a pressure limiting function is triggered. In general, the pressure sensor system must meet high requirements for accuracy and robustness, which causes high costs.

In contrast, the invention has for its object to provide a hydrostatic drive with a pressure cutoff with more stable behavior and simple calibration, and a method for calibrating this pressure cutoff.

The first object is achieved by a hydrostatic travel drive with the features of claim 1, the second by a method with the features of claim 9.

Advantageous developments of the travel drive are described in claims 2 to 8, those of the method in claims 9 to 15.

A hydrostatic travel drive has a hydraulic pump that can be coupled to a drive machine. There is one with one about the hydraulic pump Output coupling hydraulic motor of the travel drive can be supplied with pressure medium. The drive machine, for example a diesel or electric motor and / or the output can be part of the travel drive. The hydraulic pump is designed with an adjustable displacement or stroke volume, an adjusting cylinder with at least one cylinder space being provided for the adjustment thereof. The actuating cylinder, in particular its piston, can preferably be coupled or coupled to an actuating element of the hydraulic pump, the position of which determines the stroke volume. In order to act upon the at least one cylinder chamber with an actuating pressure which is effective to adjust the displacement, at least one electrically controllable pressure valve, in particular a pressure control or pressure reducing valve, is provided and assigned to the cylinder chamber. With the aim of limiting a pressure of the hydraulic pump so that it does not exceed an upper limit, for example, the travel drive has a device by means of which the signal pressure can be influenced. As a result, the influencing of the signal pressure, in particular via its influence on the stroke volume, limits the pressure. This pressure limitation by influencing the stroke volume becomes different from the pressure limitation, in which the pressure by blowing off the pressure medium via a pressure relief valve that opens at a set pressure limit is limited, referred to as pressure cutoff. According to the invention, the device is designed in such a way that the signal pressure - and consequently the pressure - can be controlled, in particular model-based, limited, and this controlled limitation can be calibrated.

Compared to conventional solutions, which are based on regulating the pressure at its limit, and according to which the pressure must be recorded or determined, and as a result of which the signal pressure is influenced in such a way that the limit is not exceeded, the solution based on the control according to the invention has pressure cutting has several advantages. For example, it is less complex and has more stable behavior, since it is less susceptible to vibrations, such as in the case of regulated pressure cut-off based on pressure sensors, or is even eliminated. A conventional solution, in which the device is designed as a hydromechanical controller, for example as a pressure-limiting valve to which pressure is applied, and when activated, a control pressure provided, from which the control pressure is reduced via the pressure valve, can be susceptible to transients which are difficult or even uncontrollable at transitions from this hydromechanical regulation of the pressure cutoff to the electronic control of the pressure valve - that is to say to the electronic pump control. In this case, there may be an uncontrolled increase in the stroke volume of the hydraulic pump. However, this problem is eliminated with the pressure cut-off which controls the invention. In addition, for example, pressure detection for the purpose of control can be dispensed with, as a result of which the hydrostatic travel drive can be designed to be less complex and less expensive in terms of device technology. Calibration via the device designed in this way, in particular if it is designed in such a way that the calibration can take place automatically, also shows a high pressure control accuracy. In addition, changes in the hydraulic pump, which occur in particular over its service life, can be compensated anew with each calibration.

Since the calibration can thus be carried out by the hydraulic pump already installed - in principle using on-board equipment - the calibration can be carried out during maintenance in the field and there is no manual adjustment or return to the factory.

In a further development, the hydraulic pump is built or configured in such a way that the pressure counteracts the adjusting effective adjusting pressure. The pressure always acts in the direction of its own reduction, as a result of which the hydraulic pump has an internal regulating effect.

In a further development, the stroke volume of the hydraulic pump can be adjusted on both sides of a zero volume or a neutral position via the actuating cylinder.

In a further development, the device is designed in such a way that it can be used to determine, in particular calculate, the stroke volume of the hydraulic pump or a variable on which the stroke volume is based - for example a swivel angle of a swash plate in the case of the hydraulic pump designed as an axial piston machine in the form of a swash plate construction - by balancing the pressure medium volume flow.

If the hydraulic motor is designed with a constant stroke volume, the current stroke volume required for balancing is the nominal stroke volume and is therefore always known. The nominal stroke volume is preferably stored in the device, in particular for the purpose of balancing.

A method according to the invention for calibrating a device for a travel drive, which is designed in accordance with at least one aspect of the preceding description, has the steps of actuating the pressure valve with an actuation current in accordance with a step-like or continuous ramp function, detecting the pressure as a function of the actuation current, and determining the one limit of the pressure or a control current associated with a cut-off pressure if the pressure exceeds the limit reached, and filing of the limit and the assigned drive current and thereby calibration of the cut-off. In this way, an automatic comparison of the behavior of the hydraulic pump in the traction drive is created, which ensures high pressure control accuracy.

The calibration or adjustment is preferably carried out when the travel drive is at a standstill.

In a further development, the calibration can be initialized by a driver or operator. As an alternative or in addition, the calibration can be initialized by a control device of the travel drive or by the device, in particular when a predetermined event of the travel drive is detected.

In one possible embodiment of the method, the limit is first explicitly specified by explicitly storing it in the device at the beginning. Then the reaching of the limit can be detected directly by the above-mentioned detection of the pressure and the then effective control current can be assigned and stored for calibration.

Alternatively or additionally, the step of determining the drive current associated with the limit or the cut-off pressure is carried out as a function of a determined characteristic pressure value of the travel drive and a pressure distance therefrom. As a result, the calibration can take place relative to the characteristic pressure value, in particular an opening pressure of a pressure limiting valve.

In further biodegradation, the method therefore has the steps of implicitly specifying the limit as a pressure offset of, in particular, a stationary opening pressure of a pressure limiting valve of the travel drive, determining the opening pressure from a course of the detected pressure (in particular from a time course, in particular from a time gradient of the detected pressure and a valve characteristic ), Determining the limit from the opening pressure and the pressure offset, and, in particular starting from the opening pressure, actuating the pressure valve with the actuating current in accordance with a falling ramp function up to the limit. As a result, the calibration is carried out in such a way that it takes place relative to the opening pressure of the pressure-limiting valve and thus maximum use is made of the available pressure. In the course of this, the setting of the pressure relief valve can also be measured. A comparison of the opening pressure and the set value then provides information on a possibly necessary correction of the setting or maintenance of the pressure relief valve.

In a further development, the step of determining the opening pressure from the course of the detected pressure comprises steps of determining an opening of the pressure limiting valve from the course of the detected pressure, and maintaining the activation current effective during opening during a period in which the pressure at the opening pressure stabilizes. The duration is known in particular from a valve characteristic of the pressure limiting valve stored in the control unit.

The limit mentioned can be, for example, the already mentioned maximum permissible pressure or a minimum necessary pressure, for example at a starting point of the travel drive at which the drive overcomes the starting resistances and starts to move.

It is fundamentally advantageous to carry out the calibration under defined conditions. To this end, a development of the method has a previous step of establishing at least one calibration condition. This is, for example, the achievement and maintenance of a speed of the hydraulic pump (or of its drive machine) that is relevant during driving operation and / or that a shaft power of the hydraulic motor is zero. The last-mentioned calibration condition is also called a block condition, since the hydraulic pump delivers against a hydraulic motor that cannot deliver shaft power. This is achieved by means of a parking brake and / or by a zero stroke volume of the hydro motor.

The calibration, determined under block condition, from reference values of the limit and assigned control current can be used in a further development to tolerate the hydraulic pump outside the block condition, in particular at a maximum performance point of the hydraulic pump with maximum stroke volume and nominal pressure. The nominal pressure is the pressure provided under normal conditions, well below the limit, for example about 200 bar.

A hysteresis of the pressure cut-off can also be determined via the calibration according to the invention and a correction factor or an offset can be determined therefrom.

The method can preferably be carried out for a forward drive operation and / or a reverse drive operation.

In addition to calibration, the method has, in a further development, steps for controlled limitation of the pressure by influencing, in particular controlling, the at least one signal pressure via the device.

In a further development, the method has a step or steps of determining a train operation or braking operation of the travel drive, and / or determining a direction of travel of the travel drive, and / or selecting the characteristic and / or characteristic map of the hydraulic pump and / or the pressure valve depending on the determined operation and / or the determined direction of travel, via the device.

In a further development, the method has a step of determining the maximum permissible signal pressure from a characteristic curve of the hydraulic pump, in which the signal pressure is described as a function of a limit of the pressure and at least as a function of a stroke volume of the hydraulic pump or a quantity of the hydraulic pump representing this stroke volume Facility.

In a further development, the method has a step of determining a necessary signal pressure according to a speed request from a characteristic map of the hydraulic pump, in which the signal pressure is described as a function of the pressure and at least as a function of a stroke volume of the hydraulic pump or a quantity of the hydraulic pump representing this stroke volume Facility.

In a further development, the method has the steps of determining a smaller and necessary maximum actuating pressure, determining an electrical control current of the pressure valve from a valve characteristic curve of the pressure valve, in which the electrical control current is described as a function of the control pressure, according to the determined lower of the control pressures, and actuation of the pressure valve with this control current, via the device.

The steps mentioned apply preferably to the pump operation of the hydraulic pump. In its engine operation, the method in a further development has the same steps with regard to the second signal pressure for acting on the second cylinder chamber.

In a further development of the traction drive and / or the method, a variable specification of the pressure and / or the limit is provided, so that a torque of the hydraulic pump and / or an output of the hydraulic pump can be controlled as a function of factors, such as driving speed, temperature or others or are.

In motor operation of the hydraulic pump, a response of pressure limiting valves can be prevented when reversing via the pressure cutoff according to the invention.

The pressure cut-off according to the invention allows braking pressure to be controlled when reversing and decelerating.

In a further development, the electronic control according to the invention can be compared with the real pump physics: for example, the hydraulic pump can be adjusted on a test bench under defined conditions, necessary control signals or currents can be determined on the test bench under defined conditions and as parameters to the control unit - In particular to their software - are transmitted, and the parameters can be automatically compared in the control unit in the sense of a calibration function.

The control according to the invention, in particular in terms of device technology and process technology, can easily be transferred to the most varied types and sizes of hydraulic pumps.

The steps are preferably automated, in particular controlled by the device.

An exemplary embodiment of a hydrostatic travel drive according to the invention and an exemplary embodiment of a method according to the invention for calibrating its pressure cut-off are shown in the drawings. The invention will now be explained in more detail with reference to the figures of these drawings.

• 1 einen hydraulischen Schaltplan eines hydrostatischen Fahrantriebes, gemäß einem Ausführungsbeispiel,
• 2 ein theoretisches Zeitdiagramm eines Drucks und eines Ansteuerstromes gemäß einem Verfahren zur Kalibrierung, gemäß einem Ausführungsbeispiel, und
• 3 ein vermessenes Zeitdiagramm einer Antriebsdrehzahl, des Drucks und des Ansteuerstromes, gemäß dem Verfahren gemäß 2.

Show it:

• 1 a hydraulic circuit diagram of a hydrostatic drive, according to an embodiment,
• 2 3 shows a theoretical time diagram of a pressure and a drive current according to a method for calibration, according to an exemplary embodiment, and
• 3 a measured time diagram of a drive speed, the pressure and the drive current, according to the method of 2 ,

According to 1 has a hydrostatic drive 1 a hydraulic pump 2 operating in a closed hydraulic circuit via the working lines 4 and 6 is fluidly connected to a hydraulic motor, not shown, for its pressure medium supply. Here is the hydraulic pump 2 with a drive machine (not shown) via a drive shaft 8th coupled to transmit a torque. The coupling is not translated, so that the speed of the drive machine and the hydraulic pump 2 are identical. The hydraulic pump 2 is designed as a swash plate type axial piston pump and can be operated in both directions of rotation and in both pump and motor operation. she has an adjustable displacement volume V _P and an adjusting device designed as a double-acting hydraulic cylinder 10 , The hydraulic cylinder 10 has a first cylinder chamber 12 and a second cylinder chamber which counteracts the first 14 , The first cylinder chamber 12 is via a first signal pressure line 16 with the output of a first pressure reducing valve 18 connected. The latter is on a control pressure line 20 connected via a control pressure connection p _s and via a feed pump 22 that are on the same drive shaft 8th how the hydraulic pump 2 sits, can be supplied with control pressure medium. The second cylinder chamber is the same 14 via a second signal pressure line 24 with a second pressure reducing valve 26 connected, which to the control pressure line 20 connected. The pressure reducing valves 18 . 26 can be operated electromagnetically, each in the signal pressure line 16 respectively. 24 resulting signal pressure p _a or p _{b is} proportional to a control current I _a or I _{b of} the electromagnet a or b according to a valve characteristic. Via the electromagnetic actuation of the pressure reducing valves 18 . 26 can thus set the actuating pressures p _a , p _{b of} the cylinder chambers by specifying the control currents I _a , I _b 12 . 14 to be controlled. For this purpose, the electromagnets a, b of the pressure reducing valves 18 . 26 via a respective signal line 28 respectively. 30 with an electronic control unit 32 signal-connected.

Furthermore, the hydrostatic travel drive 1 a speed detection unit 34 , over which a speed n _{P of} the hydraulic pump 2 detectable and via a signal line 36 to the electronic control unit 32 is communicable. The travel drive also has 1 a speed detection unit (not shown), via which the speed n _{M of} the hydraulic motor can be detected and via the signal line 38 to the electronic control unit 32 is communicable.

For safety-related pressure protection of the work lines 4 . 6 the hydrostatic travel drive has an overload 1 one pressure relief valve each 40 with the respective management 4 . 6 connected is. Both pressure relief valves 40 are with their outputs to a feed pressure line 44 connected with the feed pump 22 connected is. The feed pressure line 44 is about a throttle 42 with the control pressure line 20 fluidly connected. In the event of the pressure relief valves responding, pressure medium is thus introduced into the feed pressure line 44 relaxed, whereby energetic losses are lower than if the relaxation to tank T would happen. The pressure relief valves 40 each have a feed or suction function in the form of a check valve.

The hydrostatic drive 1 can be operated in train mode as well as in towing or braking mode. The hydraulic pump works in train operation 2 in pump mode, in braking mode, it works in motor mode. In addition, the hydraulic pump 2 reversible, that is, its displacement volume V _P is via the adjusting device 10 adjustable on both sides of a neutral position with zero volume V _P = 0. This means that the direction of rotation of the drive shaft remains the same 8th and the drive machine (diesel engine) can change direction.

The electronic control unit 32 is over a signal line 46 connected to an operator interface in the form of an accelerator pedal (not shown). A driver requests a speed from the accelerator pedal to the electronic control unit 32 transmitted. This can affect both the backward drive and the forward drive. If the accelerator pedal is operated, this corresponds to the pull or pump operation of the hydraulic pump 2 , however, if the accelerator pedal is released, this corresponds to the braking or motor operation of the hydraulic pump 2 , The actuation of a driving brake (not shown) corresponds to the braking or motor operation of the hydraulic pump 2 , The control unit is designed in such a way that it can determine the corresponding operation on the basis of the actuation mentioned. The hydrostatic drive system points to the selection of a direction of travel 1 in addition, an actuatable direction switch (not shown) on via a signal line 48 with the electronic control unit 32 is connected to the signal. The hydraulic pump is activated depending on its position 2 in its reversed or non-reversed adjustment range, that is to say on this side or beyond the neutral position of the stroke volume of the hydraulic pump 2 , The following driving conditions are defined for further consideration:

Forward travel, train operation: pressurization of the first cylinder chamber 12 via the first signal pressure line 16 and the first pressure reducing valve 18 with the first actuating pressure p _a by actuating the first pressure reducing valve 18 with the control current I _a via the control unit 32 over the first signal line 28 ,

Forward travel, braking: pressurization of the second cylinder chamber 14 via the second signal pressure line 24 and the second pressure reducing valve 26 with the second signal pressure p _b by controlling the second pressure reducing valve 26 with the control current I _b via the control unit 32 via the signal line 30 ,

Reverse travel, train operation: pressurization of the second cylinder chamber 14 over the chain 24 . 26 . 30 . 32 ,

Reverse travel, braking mode: pressurization of the first cylinder chamber 12 over the chain 16 . 18 . 28 . 32 ,

In the shown embodiment of the hydrostatic drive 1 is the hydraulic pump 2 designed such that the pressure p in the high-pressure line of the working lines 4 . 6 is present, counteracts the effective signal pressure p _a or p _b and is effective in the direction of its own reduction. For this purpose, the hydraulic pump 2 a constructively implemented control loop. In the present case, the hydraulic pump designed as an axial piston pump in a swashplate design 2 this is realized in such a way that a control disc of the hydraulic pump 2 is arranged twisted with respect to an axis of rotation of its cylinder drum. Mouths of those cylinders which are connected to the pressure kidney control disk having the pressure (high pressure) are thereby arranged asymmetrically distributed with respect to a pivot axis of the swash plate. The end sections of the working pistons guided in the cylinders are then also distributed asymmetrically. The asymmetrical supporting forces of the working pistons result in a torque which swivels back on the swash plate in pump operation and swings out in motor operation. As a consequence, a relationship arises in the form of a pump characteristic or a map of pump characteristics of the hydraulic pump 2 , in which the respective signal pressure p _a , p _b depending on the pressure p and the stroke volume V _{P of} the hydraulic pump 2 , and their speed n _P can be written. These characteristics or maps are measured and in the electronic control unit 32 for processing, in particular for executing the method described later.

The description of normal driving operation of the hydrostatic drive follows 1 , The starting point of the description is an unactuated accelerator or accelerator pedal and a drive machine rotating at idle at idle speed. The operator first actuates the accelerator pedal, which increases the speed of the prime mover (diesel) from idling to the nominal speed. Accordingly, the electronic control unit 32 depending on the speed of the diesel engine, a control signal or control current I _a for the hydraulic pump 2 , more precisely for their first pressure reducing valve 18 , When the nominal speed of the drive machine is reached, a maximum driving speed of the drive system is reached 1 receive. Accordingly, the first signal pressure p _a according to one in the electronic control unit 32 stored map of the hydraulic pump 2 elevated. Since there is no load yet, the hydraulic pump swivels 2 fully to their maximum stroke volume V _Pmax and delivers their maximum volume flow Qmax at nominal speed.

As a result of driving resistance, a pressure or load pressure p, for example 250 bar, is established when driving in one plane. An operating point is then reached which lies on a curve of maximum power P _{nomeng of} the drive _machine . At this operating point, the first signal pressure p _a at the nominal speed is dimensioned such that the hydraulic power pQ _{max of} the hydraulic pump 2 corresponds to the nominal power P _nomeng .

Now takes the load on the traction drive, for example when driving up a hill or taking gravel in the case of a wheel loader 1 too, the pressure p increases. Due to the aforementioned design of the hydraulic pump 2 , in the case of the train operation of the hydraulic pump 2 when the working pressure p counteracts the first actuating pressure p _a in the direction of a reduction in the stroke volume V _p , the pressure p swings the pivoting cradle of the hydraulic pump 2 back, which slows down the journey. The first signal pressure p _a is not changed in the meantime, so that when the pressure p increases further or a pressure difference Δp follows, the stroke volume V _P is further reduced.

The electronic control unit ensures that a maximum permissible pressure pmax or cut-off pressure or a maximum permissible pressure difference Δp _{max is reached} 32 for ensuring that this limit pmax, Δp _{max is} not exceeded. Accordingly, despite a further increasing load, there is no further increase in pressure p, since the first signal pressure p _{a occurs} via the control unit 32 via the pressure reducing valve 18 according to 1 is lowered such that the pressure p _{max is} not exceeded. So it is in the control unit, for example 32 a maximum permissible pressure p _max of, for example, 450 bar is set, the control unit intervenes 32 according to the pressure cut-off controlling the invention and reduces the first signal pressure p _a . In this way, the maximum permissible pressure p _{max can be} prevented even if the load continues to rise.

At least the following are to be mentioned as input variables of a method according to the invention: A swivel angle α _{P of} the hydraulic pump proportional to the stroke volume V _P 2 whose speed n _P , which in the exemplary embodiments is the same or proportional to the speed neng of the drive machine, and the limit pmax to be defined or predetermined of the maximum permissible working pressure, that is to say the so-called cut-off pressure.

2 shows a time diagram of a pressure curve p (t) and drive current curve I _a (t) of an embodiment of a method according to the invention. At least two exemplary embodiments of a method should be mentioned. Both have in common that calibration conditions are first created. This is done, on the one hand, by driving or setting a pump speed n _P to a value that is relevant during driving operation. This corresponds in particular, for example, the nominal engine speed neng. Add to that the hydraulic pump 2 is driven under a so-called block condition. This means that the hydraulic motor, which it supplies with pressure medium, is either set to an absorption volume V _{N of} zero or, in the case of a hydraulic motor designed as a constant machine, is locked by a brake. In both cases the shaft power of the hydraulic motor is zero and the hydraulic pump 2 only generates leakage in the hydraulic circuit due to its pump output.

In the next step, the control current I _a is raised continuously via a ramp which is designed continuously or step by step. In this way, the limit pmin or pmax for which the assigned drive _current I _{amax is} to be determined in the sense of a calibration is approached. At this point, the two exemplary embodiments are now divided into different branches.

According to a first exemplary embodiment of the method, the actuation current I _a continues to be increased until the previously explicitly defined limit or the previously explicitly defined cutting pressure p _min or p _{max is} reached and detected. The drive current I _a then specified at this point in time is then used as a reference value for the pump control in the device 32 saved. With this reference value there are tolerances of the hydraulic pump 2 and the pressure reducing valve 18 balanced. These steps are carried out analogously to the steps mentioned, which represent the train operation in forward travel, for the train operation in reverse travel.

The second exemplary embodiment of the method does not go through the detection of the pressure cut-off point (explicitly specified limit), but uses the function of the pressure limiting valve 40 , After the two steps identical to the first exemplary embodiment create calibration conditions and continuous raising of the pump control, the control current I _{a is increased in} accordance with 2 until the pressure relief valve opens 40 according to 1 is measurable at point "1". This is done via the control unit 32 the pressure p according to 2 Diagram recorded above, which is due to the gradual increase in the control current I _a 2 further increased below. As long as a gradient Δp / Δt is positive, the pressure relief valve has 40 not yet open. Detects the control unit 32 a negative gradient Δp / Δt, an opening point is “1” of the pressure relief valve 40 detected. The pressure relief valve shows from opening point "1" 40 behavior in the form of a characteristic drop in pressure p. Beyond the opening point "1", the pressure p then stabilizes in accordance with 2 above in a plateau, which is the opening pressure p _{DB of} the pressure relief valve 40 is detected and in the control unit 32 is filed.

There follows a gradual lowering of the control current I _a 2 below until, starting from the opening pressure p _DB, the pressure p has dropped to the limit pmax calculated from the pressure interval Δp _DB . Then in 2 from the control unit 32 applied control _current I _amax together with the limit pmax in the control unit 32 saved, creating tolerances of the hydraulic pump 2 and the pressure reducing valve are adjusted.

3 shows real measured time profiles of the control current I _a , the pressure p, the filtered pressure p _f , the drive speed neng as setpoint and actual value, and the drive torque Meng. The second exemplary embodiment of the method is shown with the detection of an opening point “1” and a pressure limiting point “2” at which the pressure p (p _f ) remains stationary after opening. According to 3 At the top, a _nominal speed n _{engsoll of} the drive _{machine is set} to 2000 revolutions and regulated. The actual value is small. According to 3 In the middle, the control current I _{a is} gradually increased. Accordingly, the pressure p and the filtered pressure p _f follow 3 below. At opening point "1" the pressure p and the filtered pressure p _f drop rapidly. This is done by the control unit 32 detected and recognized as opening point "1". In the control unit 32 is a characteristic of the pressure relief valve 40 deposited from which the pressure curve of the pressure relief valve 40 depending on the time t after the opening point "1" is known. The control unit 32 therefore reacts from opening point "1" by keeping the control current I _a constant, so that the pressure p or p _f can stabilize. This is the case if the gradient Δp (in particular Δp _f ) / Δt is equal to zero, which is shown in 3 the case below at the so-called pressure limitation point "2". The pressure p _DB and the associated current I _aDB are in the control unit 32 stored. About that in the control unit 32 previously stored offset, or the pressure distance Δp _DB , the limit p _amax can thus be calculated immediately. Following the plateau of the control _current I _aDB takes place via the control unit 32 now the actuation of the pressure reducing valve 18 with a control current I _a with gradually decreasing ramp function. What is striking is a renewed increase in pressure when the pressure relief valve is closed 40 , This is followed by a renewed lowering of the pressure p, p _f until the pressure p, p is the previously calculated limit _f p _amax achieved. The pair of values from p _amax and assigned I _amax is in the control unit 32 saved. So that are the tolerances of the hydraulic pump 2 and the pressure reducing valve 18 adjusted.

What is disclosed is a hydrostatic travel drive with a hydraulic pump with an adjustable stroke volume, the adjustment being carried out by means of a control pressure provided in proportion to a control current, via which a hydraulic motor can be supplied with pressure medium. According to the invention, pressure limitation or cutoff on the basis of a controlled limitation of the signal pressure, as well as automated calibration of the pressure cutoff, are provided via an electronic control unit of the travel drive.

Also disclosed is a method for calibrating said pressure cut-off via the device, with steps of driving a ramp of the drive current, detecting the pressure at the limit and the associated drive current, and storing this pair of values in the device.

Claims (15)

Hydrostatic travel drive with a hydraulic pump (2), which can be coupled to a drive machine, for supplying pressure medium to a hydraulic motor of the travel drive (1) which can be coupled to an output (80), the hydraulic pump (2) having an actuating cylinder (10) with at least one cylinder chamber (12 , 14) and an adjustable stroke volume (V _P ) and at least one electrically controllable pressure valve (18, 26) is provided, via which the cylinder space (12, 14) can be acted upon by an adjusting pressure (p _a , p _b ) , and with a device (32) by means of which a pressure (p) of the hydraulic pump (2) can be limited by influencing the signal pressure (p _a , p _b ), characterized in that the device is designed in such a way that the pressure (p) controlled limitable and this controlled limit is calibratable. Travel drive after Claim 1 wherein the device is an electronic control unit (32). Travel drive after Claim 1 or 2 , The device (32) has a characteristic curve (82, 84) of the hydraulic pump (2) in which the signal pressure (p _a , p _b ) as a function of the limit (p _max ) of the pressure (p) and at least the stroke volume ( V _P ) of the hydraulic pump (2) or a variable (α _P ) of the hydraulic pump (2) representing this displacement (V _P ). Travel drive according to one of the Claims 1 to 3 The device (32) has a characteristic map (50) of the hydraulic pump (2) in which the signal pressure (p _a , p _b ) as a function of the pressure (p) and at least the stroke volume (V _P ) of the hydraulic pump (2) or a quantity (α _P ) of the hydraulic pump (2) representing this stroke volume (V _P ). Travel drive after Claim 3 and / or 4, the control pressure (p _a , p _b ) as a function of a rotational speed (n _P ) of the hydraulic pump (2) being described in the characteristic curve (82, 84) and / or the characteristic diagram (50). Travel drive at least after Claim 3 A maximum permissible signal pressure (p _amax , p _bmax ) can be determined from the characteristic (82, 84) via the device (32). Travel drive at least after Claim 4 A control pressure required according to a speed requirement can be determined from the map (50) via the device (32). Travel drive according to one of the preceding claims, wherein the device has a valve characteristic of the pressure valve (18, 26), in which an electrical control current (I _a , I _b ) is described as a function of the signal pressure (p _a , p _b ). Method for calibrating a device of a travel drive (1), which is designed according to one of the preceding claims, characterized by steps - actuating the pressure valve (18) with an actuating current (I _a ) according to a ramp function, - detecting the pressure (p) in dependence the control current (I _a ), - determining the control _current (I _amax ) associated with a limit (p _max ) of the pressure (p) when the pressure (p) reaches the limit (p _max ), and - storing the limit (p _max ) and the assigned control _current (I _amax ). Procedure according to Claim 9 , with one step - explicit specification of the limit (p _max ). Procedure according to Claim 9 , with steps - implicit specification of the limit (p _max ) as the pressure offset (Δp _DB ) of an opening pressure (p _DB ) of a pressure limiting valve (40) of the travel drive (1), - determination of the opening pressure (p _DB ) from a course of the detected pressure ( p), - Determination of the limit (p _max ) from the opening pressure (p _DB ) and pressure offset (Δp _DB ), and - Control of the pressure valve (18) with the control current (I _a ) according to a decreasing ramp function, up to the limit (p _max ). Procedure according to Claim 11 , the step of determining the opening pressure (p _DB ) from the course of the detected pressure (p), steps - determining an opening of the pressure relief valve (40) from the course of the detected pressure (p), and - maintaining the effective control current when opening (I _a ) during a period (t _DB ) in which the pressure (p) to the opening pressure (p _DB ) has stabilized. Procedure according to Claim 11 or 12 , with one step - Adjust the opening pressure (p _DB ) against an opening pressure (p _DBset ) set on the pressure relief valve (40). Procedure according to one of the Claims 9 to 13 , the limit being a maximum permissible pressure (p _max ) or a minimum necessary pressure (p _min ) of driving operation of the travel drive (1). Procedure according to one of the Claims 9 to 14 with a previous step - establishing at least one calibration condition.

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